Antibacterial antibiotics have their effect by blocking one
or another of the bacterial cell's synthetic pathways. The
penicillins for example block cell wall synthesis, the
streptomycins and kanamycins interfere with protein
synthesis, etc.
The few antiviral antibiotics which are available work in a
similar manner. However, since most of the synthesis of the
components of most viruses is carried out by host cell
enzymes, finding non-toxic antivirals is difficult.
Bob S.